For nearly a decade the bidirectional thyristor, or TRIAC, has been replacing the relay as a means for controlling the application of AC power to loads such as incandescent lamps or heating elements in response to an electrical control signal. The TRIAC is nearly ideal for this purpose, offering the combination of small size, high efficiency, solid state reliability, and the ability to be controlled by signals as much as 5 orders of magnitude smaller than those required for a relay of similar capacity.
The TRIAC is connected in the same way as a mechanical switch; that is, one side of load is connected directly to one power line and the other side is connected to the remaining power line through the TRIAC. Power to control the TRIAC and for any associated circuitry (e.g., thermocouple amplifiers and limit detectors) is obtained by connecting a power supply across the power lines, using a transformer or a resistor to drop the voltage to a useable level.
While the requirement for a direct connection across both power lines rarely causes problems in new installations, when the TRIAC is used to replace a previously installed switch or thermostat it can create difficulties. Since neither switches nor mechanical thermostats require a connection to both power lines, very often the power line connected directly to the load is routed by the shortest path, and may come nowhere near the switch box or thermostat. This is particularly true of residential light switches, where in order to make a connection to both power lines in a switchbox it is often necessary to remove a section of wall, and install a special wire. This is clearly impractical in most cases, and as a result, with the exception of lamp dimmers the TRIAC has found little use as a replacement for residential light switches.
The availability of a power supply that did not require a connection to both power lines would make it possible to install devices such as proximity switches, touch switches, timers, and electronic thermostats as direct replacements for mechanical switches. The fact that such devices are not yet in wide use is due principally to difficulties encountered in bringing both power lines to the switchbox for installation.